A stately home which is said to be the largest house in Britain will be sold to a preservation group dedicated to restoring some of its former glory.

The sale of Wentworth Woodhouse has been agreed for £7 million, although it will take tens of millions more to repair and refurbish the country mansion.

The massive house, near Rotherham, is said to have 365 rooms and five miles of corridors. Many of its opulent rooms are now empty and most of its treasures – including the famous 18th Stubbs painting Whistlejacket – have been moved to new homes.

The house went up for sale last year following the death of owner Clifford Newbold. Mr Newbold’s family agreed to sell to an investment company last year but the deal fell through.

Now the Wentworth Woodhouse Preservation Trust (WWPT) has confirmed it has agreed to buy the mansion. The trust has extensive restoration plans for the building over the next 15 years.

Small pre-booked tours were already run at Wentworth Woodhouse by the Newbolds, but the trust is hoping to open up large parts of the property to the public, with the help of the National Trust, and convert other sections for residential development and an events venue. SAVE Britain’s Heritage, which has supported the trust, said the sale should be completed within three months.

In a statement, it said: “SAVE is delighted to announce that agreement has been reached on the purchase of one of the finest and grandest historic houses in Britain, Wentworth Woodhouse.”

It said the £7 million cost of the acquisition has been partly funded by a £3.5 million grant from the National Heritage Memorial Fund. The rest of the cash has come from a range of institutional and private donors.

Wentworth Woodhouse was the northern seat of the Fitzwilliam family – at its height one of the richest and most powerful aristocratic dynasties in England.

After the family moved out 40 years ago, the building was used as a teacher training college before it passed into private ownership.

The house has been ravaged by problems relating to subsidence from the coal workings beneath the surrounding park.

Julie Kenny, chair of WWPT, said: “It’s a fantastic house that needs a lot of work doing on it, but it’s absolutely worth it because there’s no house like it anywhere in the UK.

“It’s a Grade 1 listed building which is steeped in South Yorkshire history. It’s had two prime ministers live there and five MPs. We were raising the money to buy it and now we are raising the money to renovate it.”

Visit www.justgiving.com/savewentworthwoodhouse to donate.